1.3.8.2 North-east Atlantic extra-tropical cyclones

The North-east Atlantic is the region with the deepest observed central pressures of extra-tropical cyclones, and the adjacent margin of north-west Europe has the greatest levels of extra-tropical cyclone historical building damage, forestry windthrow, and storm-surge impacts observed worldwide. Many studies report an increase in the 1980s in the number of deep (and high wind-speed) extra-tropical cyclone storms in this region (see Günther et al., 1998) returning to levels not previously seen since the late 19th century (see Trenberth et al., 2007, Section 3.5.3). Various measures, including increases in the number of deep storms (with central pressures less than 970 hPa) and reductions in the annual pressure minimum of storms crossing the Greenwich Meridian all show evidence for intensification, in particular between 1980 and 1993, when there were a series of major damaging storms. In the North-east Atlantic, wave heights showed significant increases over the period from 1970 to 1995 (Woolf et al., 2002) in parallel with the NAO index, which reached its highest values ever (reflecting deep low pressure over Iceland) in the years of 1989 to 1990. Intense storms returned at the end of the 1990s, when there were three principal damaging storms across western Europe in December 1999. However, since that time, as winter NAO values have continued to fall (through to March 2005), there has been a significant decline in the number of deep and intense storms passing into Europe, to some of the lowest levels seen for more than 30 years. (Other high-latitude regions of extra-tropical cyclone activity also show variations without simple trends: see Trenberth et al., 2007, Section 3.5.3.)

1.3.8.3 Tropical cyclones

While overall numbers of tropical cyclones worldwide have shown little variation over the past 40 years (Pielke et al., 2005), there is evidence for an increase in the average intensity of tropical cyclones in most basins of tropical cyclone formation since 1970 (Webster et al., 2005) as well as in both the number and intensity of storms in the Atlantic (Emanuel, 2005), the basin with the highest volatility in tropical cyclone numbers (see Trenberth et al., 2007, Sections 3.8.3 and 3.8.3.2).

Although the Atlantic record of hurricanes extends back to 1851, information on tracks is only considered comprehensive after 1945 and for intensity assessments it is only complete since the 1970s (Landsea, 2005). From 1995 to 2005, all seasons were above average in the Atlantic, with the exception of the two El Niño years of 1997 and 2002, when activity was suppressed – as in earlier El Niños. The number of intense (CAT 3-5 on the Saffir-Simpson Hurricane Scale) storms in the Atlantic since 1995 was more than twice the level of the 1970 to 1994 period, and 2005 was the most active year ever for Atlantic hurricanes on a range of measures, including number of hurricanes and number of the most intense CAT 5 hurricanes.

In the Atlantic, among the principal reasons for the increases in activity and intensity (Chelliah and Bell, 2004) are trends for increased sea surface temperatures in the tropical North Atlantic. The period since 1995 has had the highest temperatures ever observed in the equatorial Atlantic – “apparently as part of global warming” (see Trenberth et al., 2007, Section 3.8.3.2). The first and only tropical cyclone ever identified in the South Atlantic occurred in March 2004.

While other basins do not show overall increases in activity, observations based on satellite observations of intensity (which start in the 1970s) suggest a shift in the proportion of tropical cyclones that reached the higher intensity (CAT 4 and CAT 5) from close to 20% of the total in the 1970s rising to 35% since the 1990s (Webster et al., 2005). Although challenged by some climatologists based on arguments of observational consistency, as quoted from Trenberth et al., (2007) Section 3.8.3) “the trends found by Emanuel (2005) and Webster et al. (2005) appear to be robust in strong association with higher SSTs”. Increases in the population of intense hurricanes in 2005 created record catastrophe losses, principally in the Gulf Coast, USA, and in Florida, when a record four Saffir-Simpson severe (CAT 3-5) hurricanes made landfall, causing more than US$100 billion in damages with almost 2,000 fatalities.